It is known that colorectal cancer is the second most common cause of cancer deaths in the United States and Europe. Screening for colorectal cancer can result in a significant reduction in cancer mortality. The currently most popular exam for visualizing the colon is a colonoscopy, which is an invasive and painful exam. A colonoscopy may be performed to screen for colorectal cancer as well as a number of other colon pathologies, including Crohn's disease and irritable bowel syndrome. However, due to the nature of the exam, its screening recommendation is often ignored by the public at large.
As known to those skilled in the art, a colonoscopy generally refers to a medical procedure for examining a patient's colon to detect abnormalities such as polyps, tumors or inflammatory processes in the anatomy of the colon. The colonoscopy consists of a direct endoscopic examination of the colon with a flexible tubular structure known as a colonoscope, which has fiber optic or video recording capabilities at one end thereof. The colonoscope is inserted through the patient's anus and directed along the length of the colon, thereby permitting direct endoscopic visualization of colon polyps, e.g., abnormal growths in the colon, and tumors, and in some cases, providing a capability for endoscopic biopsy and polyp removal.
Although colonoscopy provides a precise way of colon examination, it is generally time-consuming, expensive to perform, and requires great care and skill by the medical examiner. In addition, such procedure may require a thorough patient preparation, including a ingestion of purgatives and enemas, and usually a moderate anesthesia. Since colonoscopy is an invasive procedure, there may likely be a significant risk of injury to the colon and the possibility of colon perforation and peritonitis, which can be fatal.
To overcome these drawbacks, the virtual colonoscopy procedure was developed. For example, a virtual colonoscopy makes use of images generated by computed tomography (“CT”) imaging systems, which can also be referred to as computer assisted tomography (“CAT”) imaging systems. In a CT or CAT imaging systems, a computing device may be used to produce an image of cross-sections of regions of the human body by using measure attenuation of X-rays through a cross-section of the body. In a virtual colonoscopy, the CT imaging system usually generates two-dimensional images of the internals of the intestine. A series of such two-dimensional images can be combined to provide a three-dimensional image of the colon.
While this CT or CAT procedure does not require an insertion of an endoscope into a patient and thus avoids the risk of injury to the colon and the possibility of colon perforation and peritonitis, such procedure still may require a thorough patient preparation, including the use of purgatives and enemas. Generally, the patient should stop eating, and purge the bowel by ingesting (typically by drinking) a relatively large amount of a purgative. Another problem with the virtual colonoscopy approach is that the accuracy of examinations and diagnosis using virtual colonoscopy techniques is not as accurate as is desired. This is due to, at least in part, the relatively large number of images the medical examiner (e.g., a doctor) should examine to determine if a polyp, tumor and/or an abnormality exists in the colon.
Recent advances in the virtual colonoscopy space have suggested that image processing techniques may be used to remove the need for bowel cleansing prior to the procedure. For example, U.S. Pat. No. 6,947,784 and U.S. Patent Publication No. 2005/0107691, the entire disclosures of which are incorporated herein by reference, describe a procedure in which bowel contents are tagged with contrast agents ingested by the patient and eliminating the tagged bowel contents with electronic cleansing (“EC”) techniques.
The EC techniques disclosed in one or both of these publications involve a combination of thresholding and filtering techniques. An exemplary procedure provides that a global threshold is initially set to the tagged bowel contents. For example, the pixels in a CT image having an attenuation higher than the threshold are then subtracted from the image. Further, a smoothing filter may be used to average the abrupt transitions between the native and subtracted regions in the image.
Studies have shown that EC techniques increase the diagnostic ability of CT colonoscopy. See, for example, Pickart, P. and Choi, J., “Electronic Cleansing and Stool Tagging in CT Colonography: Advantages and Pitfalls with Primary Three-Dimensional Evaluation,” Am. J. Roentgenol., Sep. 1, 2003, 181(3):799-805, the entire disclosure of which is incorporated herein by reference. However, the current EC techniques are limited in that they may not be capable of removing all of the bowel contents that are not a result of or associated with an abnormality.
In particular, current EC techniques may not be able to handle three types of artifacts that are visible in the CT images after EC is performed: (i) pseudo-polyps, (ii) under-cleansing artifacts, and (iii) degraded folds.
Pseudo-polyps may appear as true polyps, but they generally arise out of partial volume effects between air and tagged bowel contents. Current threshold-based methods may not cleanse bowel contents completely. For example, FIG. 1 shows that after a performance of an exemplary conventional EC procedure so as to cleanse the tagged bowel contents 110 in a CT image 100, a pseudo-polyp artifact 115 that remained in the CT image 105.
Under-cleansing artifacts generally arise due to the heterogeneity of the tagging contrast, which tends to result in low attenuation materials that may not be removed via thresholding. For example, FIG. 2 shows that after EC was performed to cleanse the tagged bowel contents 210 in the CT image 200, a heterogeneously opacified colonic contents 215 remained in the CT image 205.
Using the conventional EC procedures. degraded folds, which generally appear as haustral folds surrounded by tagged bowel contents, may be inappropriately removed or obscure mucosal polyps due to over-segmentation of the tagged bowel contents and potential pseudo-enhancement affecting these extremely thin structures. For example, FIG. 3 shows that after EC was performed to cleanse the tagged bowel contents 310 in CT image 300, the degraded folds 315 remained in the CT image 305. Due to a pseudo-enhancement, a part of the thin fold provided next to the bowel contents 310 was inappropriately removed, resulting in the degraded fold 315 in the CT image 305.
These artifacts appear visually distracting, therefore limiting the usefulness of the currently-available EC techniques for clinical interpretation of CT colonography images. To address these concerns, certain image processing techniques have been developed to remove the artifacts which remain after the EC procedure is performed.
For example, to address the partial volume effects that may result in pseudo-polyps and other artifacts, a technique was developed and disclosed in Chen, D., et al., “A Novel Approach to Extract Colon Lumen from CT Images for Virtual Colonoscopy,” IEEE Trans. on Medical Imaging, Vol. 19, pp. 1220-26, 2000, the entire disclosure of which is incorporated herein by reference. The technique described in this publication involves Markov random field to characterize each voxel by its local feature vector.
To address the over-segmentation problem, Zalis et al., “Digital Subtraction Bowel Cleansing for CT Colonography Using Morphological and Linear Filtration Methods,” IEEE Trans. on Medical Imaging, Vol. 23, pp. 1335-43, 2000, the entire disclosure of which is incorporated herein by reference, suggests using an edge filter to identify regions with artifacts and add the over-subtracted regions back before smoothing.
Both of these image processing techniques employ thresholding and region growing to segment tagged bowel contents. However, they still cannot solve the problems caused by the heterogeneity of the tagged bowel contents (shown in FIG. 2) and the problems caused by degraded folds (shown in FIG. 3).
There is a need to overcome the deficiencies described herein above.